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A star transitions into a supernova when it exhausts its nuclear fuel, leading to a catastrophic collapse of its core.
A star's life is a delicate balance between the inward pull of gravity and the outward push of pressure from nuclear fusion reactions in its core. As long as there is enough nuclear fuel, the star remains stable. However, when a star exhausts its nuclear fuel, this balance is disrupted. The core of the star collapses under its own gravity, triggering a supernova explosion.
The process begins when a star has burnt through all of its hydrogen fuel and starts to burn helium. This process continues with heavier and heavier elements being formed and burnt in the core, until iron is formed. Iron is the end point of the process because it cannot be burnt to release energy. At this point, the core of the star is essentially an iron sphere, and no more energy can be generated to support the star against its own gravity.
The iron core collapses in a matter of seconds, and the outer layers of the star fall inwards. The collapsing core then rebounds, sending a shock wave outwards that rips through the falling material. This shock wave, along with a burst of neutrinos, causes the star to explode in a supernova.
The energy released in a supernova explosion is immense, briefly outshining an entire galaxy. The remnants of the star can form a neutron star or a black hole, depending on the mass of the original star. The explosion also disperses heavy elements into space, which can later form new stars and planets. In fact, most of the elements heavier than iron in the universe are thought to have been produced in supernova explosions.
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